AN AMERICAN tennis player has sent cries of anguish echoing through Wimbledon after she suffered a horrific on-court injury on Thursday.

US star Bethanie Mattek-Sands was in the third and final set of her second-round singles match against Romanian Sorana Cirstea when her leg buckled and she fell to the ground in pain.

The 32-year-old lay on her side clutching her right knee for a few moments before she began to scream in agony.

"Help me! Help me please! I need help please! F***!" she howled, as the horror incident played out on live TV.

The crowd looked on stunned as her opponent jumped the net to help her out. Cirstea was later seen in tears.

Mattek-Sands' husband Justin Sands tended to her as she lay on the turf of court 17, but it was nearly 20 minutes before medical staffers removed her via a stretcher to a waiting ambulance, the New York Post reported.

"F*** it hurts, f*** me it hurts. Have you given me any f***ing painkillers? These painkillers are not working," she was heard to say before she was taken off the court, according to The Sun.

"Please knock me out, please knock me out."

The injury to her knee was so gruesome that she asked the medics to cover it up with a towel.

After they attempted to move Mattek-Sands onto her back, she reportedly yelled: "F***ing put me down, f*** it hurts. I'm going to kill you all."

Her doubles partner Lucie Safarova was caught on camera on the sidelines in tears as she watched Mattek-Sands in agony.

Cirstea has questioned why it took the medics so long to tend to her opponent, according to The Mirror.

"I went over and saw her knee out and turned around and told them to bring a stretcher because I think everyone froze," she said.

"It was heartbreaking because she went into shock and it took a while until the medical team was there … Her knee was in a very weird position. I have never seen anything like this except in the movies. I panicked a little bit as well and called for help but no one was coming and I tried to comfort her as much as I could but you could feel the pain.

Bethanie Mattek-Sands in agony as medics tend to her. Picture: David Ramos/Getty Images

"She went into shock and saying 'Sorana help me, Sorana help me, Sorana help me' and I said 'I am here, you are strong you can do this' but I felt useless because, in this moment, it was a very uncomfortable moment and I felt useless."

Wimbledon defended its response time in a series of tweets, saying a paramedic made it onto court 17 within one minute.

AELTC statement (1/3): The first response to Court 17 was within 1 minute, by a qualified ambulance technician

Mattek-Sands, a wildcard singles player, was within reach of a win before the injury. She had won the first set 6-4 but lost the second 7-6 after a tiebreak.

The injury occurred in the first game of the deciding set.

Mattek-Sands and Safarova are the reigning doubles champions in three of the four majors and they were vying for a non-calendar-year Grand Slam at Wimbledon. Her injury is likely to put that goal out of reach.